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Silicon detectors in HEP

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  1. Silicon detectors in HEP Bluffing your way into particle physics detectors • Introduction • Semi-conductor physics • Real Si detectors • Radiation damage in Si • Radiation hard sensors • Novel devices/State-of-the-Art In case of any questions: Jaap Velthuis (University of Bristol)

  2. Introduction • Particle Physics is more than hunting for Higgs and CP violation • Need to make very advanced detector systems • Forefront of • Engineering (stiff light weight support structures, cooling, tunnel building) • High speed and radiation hard electronics • Computing (web, grid, online) • Accelerators (e.g. cancer therapy, diffraction) • Imaging sensors (e.g. nth generation light source, medical imaging) Jaap Velthuis (University of Bristol)

  3. Introduction • Why semi-conductor devices • P-N junction • Particle traversing matter • Scattering • Signal generation • Summary • Baseline detector Jaap Velthuis (University of Bristol)

  4. Why semi-conductor devices Jaap Velthuis (University of Bristol)

  5. Standard experiment Jaap Velthuis (University of Bristol)

  6. The “Onion” peeled… Electro- magnetic calorimeter Muon chamber Very precise tracking hadronic calorimeter tracking Outward Track density drops • Fundamental parameters: • Charge • Momentum • Decay products • Life time • Decay vertex • Mass • Spin • Energy • Need very precise tracking close to primary vertex. Then follow track to calorimeter and measure energy. Jaap Velthuis (University of Bristol)

  7. Tracking R- Secondary vertex Primary vertex • Track described by 5 parameters • Modern tracking uses “Kalman Filter” • Start with “proto” track • Add new point • Update 2 • Decide to in- or exclude point based on 2 • Modern Vertexing • Use tracks with errors • Add them to vertex • Calculate 2 etc • So to do good tracking and vertexing, need detectors with small error and little “deflection” Jaap Velthuis (University of Bristol)

  8. Wire chambers • Traditionally tracking in wire chambers Jaap Velthuis (University of Bristol)

  9. Wire chambers • Problem in wire chambers: • Wires long • Many hits per wire for wires close to primary vertex (high occupancy) • Leads to ambiguities in track fitting • Solution: very short wires!  solid state Jaap Velthuis (University of Bristol)

  10. Charged particle traversing matter Atomic number/mass absorber Electric charge incident particle Some constant Maximum kinetic energy which can be imparted to a free electron in a single collision Mean excitation energy • Energy loss described by Bethe-Bloch equation: Jaap Velthuis (University of Bristol)

  11. Charged…matter • dE/dx different for different particles due to different M and  • Is used to identify different particles Jaap Velthuis (University of Bristol)

  12. Charged … matter Relevant for detectors dE/dx [MeV cm2/g] • Energy loss wildly varying function, • MINIMUM IONIZING PARTICLE (4) Jaap Velthuis (University of Bristol)

  13. Charged … matter • Bethe-Bloch describes average energy loss • Collisions stochastic nature, hence energy loss is distribution instead of number. • First calculated for thin layers was Landau. Hence energy loss is Landau distributed. • Signal proportional to energy loss  is most probable value Jaap Velthuis (University of Bristol)

  14. Multiple scattering • During passage through matter Coulomb scattering on nucleideviation from original track • Deflection distribution Gaussian with width 0 • More dense material, more scattering, shorter X0 Jaap Velthuis (University of Bristol)

  15. Detector trade off • Thick detectors (in X0) lots of energy lost  lots of signal generated • But loads of scattering  bad for tracking • Loads of -electrons more signal but not right direction Jaap Velthuis (University of Bristol)

  16. Silicon trivia • Silicon was discovered by Jöns Jacob Berzelius in 1824 • Name from “silicis” (Latin for flint) • With 25.7% second most abundant element in earth’s crust • First crystalline silicon produced by Deville in 1824 Jaap Velthuis (University of Bristol)

  17. Why solid state detectors • Small band gap  • low energy required for e-h pair (3.6 eV in Si ~30 eV gas) • Many e-h pairs per unit length (80/m in Si) • High density  • Large energy loss per unit length  • Can make thin detectors with high signal • Small range for -electrons  • Very good spatial resolution Jaap Velthuis (University of Bristol)

  18. Why solid state detectors • Electron and hole mobility very high • Fast charge collection (~10 ns) • Excellent rigidity  • Self-supporting structures • Possibility of creating fixed space charge by doping Jaap Velthuis (University of Bristol)

  19. Intrinsic semi-conductors • Single non-interacting atom has set of well-defined energy levels • When forming crystals, levels undergo minor shifts resulting in bands • Probability for e- to occupy state given by Fermi-Dirac function Jaap Velthuis (University of Bristol)

  20. Intrinsic semi-conductors • Density of states • Density of free electrons n given by product Fn(E) and density of states Jaap Velthuis (University of Bristol)

  21. Intrinsic semi-conductors • Typically for intrinsic Si carrier density at 300K ~1010 cm-3 suppose strip 20m wide, 10 cm long, sensor 0.3 mm thick  S/N=410-3 • Re-writing concentrations: • Concentrations highly dependent on T and material • So three solutions: • Use high EG material • Cool device down (ni at 77K ~10-20) • Remove mobile carriers Jaap Velthuis (University of Bristol)

  22. “Trick”: doping N-type P-type Intrinsic • By introducing atoms with different number of valence e- can change number of free carriers • E.g. P, As: 5 valence e-; donor (n-type) • E.g. Al, B: 3 valence e-; acceptor (p-type) • Activation energies ~0.04eV<<EG=1.12eV Jaap Velthuis (University of Bristol)

  23. PN-junction • Holes in p-type recombine with e- in n-type, creating zone without mobile carriers (depletion) • Depleted silicon ideal for detector. Same signal, but no background! • Note: • Holes move towards p-type • Electrons move towards n-type Jaap Velthuis (University of Bristol)

  24. PN-junction • Can express as function Vbias: • Vjunc • Depletion width • Cjunc Jaap Velthuis (University of Bristol)

  25. PN-junction • By biasing detector, the depletion width can be extended over entire thickness of detector (full depletion). • Important: PN junction itself is located at interface between p-strips and n-bulk. Depletion region grows from PN junction towards n-type bias contact. • Typical values for full depletion 10-100 V before irradiation. Jaap Velthuis (University of Bristol)

  26. Depletion voltage • Bias voltage very important: • Creating large depletion zone • Signal proportional depletion thickness • Depletion zone also reduces background • Isolating strips from each other • Separating e-h pairs • Depletion voltage obtained from C-V curve Jaap Velthuis (University of Bristol)

  27. C-V curve Depletion voltage • Re-write C(Vbias) relation • Plotting 1/C2 vs Vbias yields: • depletion voltage • Doping concentration (for asymmetric doping) Jaap Velthuis (University of Bristol)

  28. I-V curves • Measure I-V to check long term stability of sensors and maximum Vbias • Note current NOT zero (leakage current) • If Vbias too large, get high currents (breakdown) • Zener breakdown • Tunnelling from occupied state in p side valence band to n side conduction band • Avalanche breakdown • Carriers from leakage current get so much kinetic energy that due to collisions new free carriers are generated Jaap Velthuis (University of Bristol)

  29. Signal generation Carriers carry kinetic energy 3/5EG Energy transferred to lattice r10 ERaman0.165 eV Taken from • Lost energy converted into free carriers • Energy needed to generate 1 e-h pair in Si is 3.6 eV • Results in 8900 e-h pairs per 100 m Si for a MIP • Charge cloud Gaussian with 10m • E-h pairs might recombine, need (strong) field to prevent this signal loss Jaap Velthuis (University of Bristol)

  30. Baseline detector • Need many diodes (here p-strips to n-bulk) • Need reverse bias to • Deplete entire sensor • Separate e-h • Need to readout signals from p-strips • Design issues: • Thick  large signal • Thin  less scattering • Thin  lower depletion voltage • Short strips less ambiguities • Strips close  very precise measurement impact position • Strips far apart  less electronics hence less expensive Occupancy: fraction of strips that has been hit Jaap Velthuis (University of Bristol)

  31. Real detectors • Real sensors have much more features: • Backplane contacts • Guard rings • Bias resistors • P-strips • Al readout strips • Coupling capacitors • … • Typical scale: • Sensors 6x6 cm • Pitch ~100 m • 512 Al strips Jaap Velthuis (University of Bristol)

  32. Charge collection • Determined by • Spatial distribution of generated charge • Field strength • Accelerates carriers in field direction • Determines time charge is moving • Separation of e-h pairs • “Horizontal” movement through diffusion • Hall effect Jaap Velthuis (University of Bristol)

  33. Charge collection • If pitch > charge cloud all charge collected on 1 strip • In this case analog signal value not importantchose digital or binary readout • To do better need to share charge over more strips need pitch20m for 300 m thick sensor • Problem: connecting all strips to readout channel yields too many strips Jaap Velthuis (University of Bristol)

  34. Summary • Semiconductor detectors are used close to primary vertex to • Limit occupancy and reduce ambiguities • Give very precise space point • Energy loss described by Behte-Bloch equation • Minimum ionizing particle • Energy loss (=signal) is Landau distributed • Particles scatter in matter, so need to have thin detectors • MIP yields 8900 e-h pairs per 100 m Si Jaap Velthuis (University of Bristol)

  35. Summary (II) • Need trick to remove free charge carriers • Use high band gap semiconductor • Cool to cryogenic temperatures • Build p-n junction and deplete detector • If pitch ~ charge cloud, charge is shared. Need lots of strips. • Trick intermediate strip using C-charge sharing, but non-linear charge sharing Jaap Velthuis (University of Bristol)